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Incorrect Z-Axis zero/home position
« on: June 20, 2017, 01:40:33 PM »
Hey folks,

First post here on the forum.  New to Mach3 and had no luck with a search - although I've had great results researching here in the past.  Hoping someone might point me in a direction for checking the issue I'm having.

I am running a DIY built machine with THK linear rails/trucks, ballscrews and Nema 23 motors.  I have measured backlash to be in the vicinity of about .02mm (no compensation applied in Mach3).  The machine is set up with homing switches on the X,Y & Z axis.  When I first start the machine, I home it using the "ref all" button, with all axis traveling to the home switches and backing off correctly.  I then jog to a position and set/zero the work coordinate G53 position (Z-axis done with a touch plate).   

The problem I have is when I jog away from the WCS zero, then hit "Go to Zero" button the Z-axis returns to a location slightly lower than it should (Go to Zero move always includes the Z-axis traveling up to the machine zero first, then back down to the WCS zero).  I narrowed down the issue by doing this repeatedly - each time hitting the "Go to Zero" button - triggering the Z-axis to travel to the machine zero and then back to the WCS zero.  Eventually I noticed that each time it returned to the WCS zero, it was a little lower than the last time.

I tested the issue by measuring the discrepancy of distance traveled at both the bottom (by locating to WCS zero) and top (by locating to machine zero) of the Z-axis with a dial indicator.  I made sure to remove any backlash before beginning to travel up to the machine zero.  It appears to be incrementally off by about .07mm each pass.  By repeatedly locating to the work zero and then MDI G28 Z0, back and forth, the axis is clearly not reaching the same machine zero location at the top, always stopping shy of the previous location that the machine zeroed to during the Homing operation.  So it seems that traveling between the WCS zero and the machine zero is pretty consistently the same distance, but always offset by a bit.  This puts the cutter too low - and even lower each time the machine raises up and back down to the WCS zero.  Hope this makes sense. 

If you need clarification, please let me know.  Any thoughts on what to test next?  Is this a software or hardware issue?

Thanks in advance for any help, tips and/or tricks,

Scott

Offline RICH

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Re: Incorrect Z-Axis zero/home position
« Reply #1 on: June 20, 2017, 05:50:06 PM »
Check / confirm current axes calibration using the Calibration feature under ALT C  tab. Maybe off.......

RICH
Re: Incorrect Z-Axis zero/home position
« Reply #2 on: June 20, 2017, 06:05:41 PM »
Thanks so much Rich.

When I thought more about it after posting, I thought the same thing - why do I always think of these things right after I walk out the door??? :).

I did test calibration this morning and made some very slight adjustments - but only using travel of 1.5" which may be leaving some errors in the longer throw I have on the Z-Axis.

I will do a more thorough calibration tomorrow when I get back to the shop. It does seem like the most logical cause.

Appreciate the reply. I'll post back any results I get.

Offline RICH

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Re: Incorrect Z-Axis zero/home position
« Reply #3 on: June 20, 2017, 06:18:51 PM »
When calibrating an axis you should use the LONGEST move possible. That way any error would  be small.
Make sure you remove the backlash before calibrating the axis.

Quote
why do you always think of these things right after you walk out the door???


It's caused by Adult CNC ADD or how you trained the thought process when mom told you something OR wife said something.
Research has shown it can also be caused by other things, but, only you would know them! ;) ::)

We try to answer all questions .....some are challenging and require additional info........in your case lets not go there.... ;D

RICH
Re: Incorrect Z-Axis zero/home position
« Reply #4 on: June 20, 2017, 07:24:55 PM »
Ha!  Seems like I have some recovering to do from CNC ADD :)

I did originally do the X & Y axis with the longest controlled measurement I could get. The Z was done early on as well - and the short check today was just to make sure it wasn't terribly off. But it looks like I need to get back on that Z-axis with a longer measurement to rule that out. Let's hope it's something that simple.
Re: Incorrect Z-Axis zero/home position
« Reply #5 on: June 21, 2017, 09:27:31 AM »
Well, I was hoping for an easy fix, but after redoing the Z-axis calibration it is still doing the exact same thing.  The calibration is now spot on for 4 and 5 inches (the longest travel I could measure safely).  I made sure to have backlash removed, and repeated the calibration tests several times to ensure it was correct. 

I have tried a few other tests; changing the safe-Z setting from machine coordinate zero (roughly 200+ mm travel for the tests) to just 10mm and 100mm.  The lesser the travel, the lesser the discrepancy.  But there IS still a discrepancy, incrementally worse each time I repeat the process.  The amount that it is off is determined by the distance traveled - the further the travel, the further it's off. 

So for example: if the Z-Axis moves up 100mm it's off by about .2mm.  If the Z-Axis only moves up by 50mm, it's off by about .02  With the 100mm travel test, for each of these runs it returns to yet another increment of .2mm off.  So after the second run of 100mm, it's off by .about .4.  Third time is off by .6 etc.  It's not 100% consistent - but often times is very close.

I also tried slowing the velocity and acceleration way down, thinking that maybe the motor or coupler was skipping/slipping - but this did not seem to help.  I have assumed that it would not be motor missed steps or the coupler slipping, because I don't think it would be so consistently off each move.   So it appears that regardless of how fast or slow it tries to get there, it always travels in the negative direction by slightly too short an amount.  And then incrementally does this each time I repeat the process.

I'm at wits end here on what to test....

Thanks for your patience.

Scott

Offline RICH

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Re: Incorrect Z-Axis zero/home position
« Reply #6 on: June 21, 2017, 08:55:22 PM »
Post some pictures of your machine, in particular the z axis.

RICH
Re: Incorrect Z-Axis zero/home position
« Reply #7 on: June 21, 2017, 09:13:16 PM »
I ended up having some odd issues with the machine today. Not sure if they're related, but I decided to reinstall the Mach3 software. I was afraid that I may have messed something up by installing, opening and then deciding against a couple different screen sets. Unfortunately, the reinstall is still showing the same issues.

Not sure if I should have started from scratch and not used my old custom XML file???

I only have a couple shots on my phone right now. I can post some more details if you want to see a specific part of the Z-Axis.



« Last Edit: June 21, 2017, 09:16:07 PM by motopreserve »

Offline RICH

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Re: Incorrect Z-Axis zero/home position
« Reply #8 on: June 21, 2017, 09:46:11 PM »
Thanks, just what I wanted to see.
1.The motor shaft coupling used on the Z "may" be the problem or contribute to it. I have experienced repeatability errors with that kind
of coupling over time. I use direct coupling with a preloaded bearing to keep any force from going back into the stepper motor
( they just have Bellvue washers  internally  ). So the suggestion would be to make a rigid coupling and test to find out.
 I am thinking it is the problem.......

2.Is the upper support  bearing preloaded to eliminate play? I preloaded mine enough such that there was no play and in doing so tried to minimize the amount of torque required to just turn the screw with no play. Was easy for me as I have torque gauge to do measurements.
First do #1 above.

3. Nest comes the ball screw nut, but, think 1 or 2 will do it.........

4. Profile you Z axis to check accuracy.  Lets not go there for now.

RICH
« Last Edit: June 21, 2017, 09:54:33 PM by RICH »
Re: Incorrect Z-Axis zero/home position
« Reply #9 on: June 21, 2017, 10:11:20 PM »
Thanks once again Rich. Very much appreciate all your help.

These couplers were not my first choice, and I have been planning on making some new ones on the lathe as soon as I waded through all the other things on the list. Seems like they just jumped to the top of the list!

My plan was for Oldham style. Do you have an example of the type you made?

I'm not sure if the bearing is preloaded an adequate amount. I do not have torque gauges to check. When I assembled the axis I made sure it did not have play - but that was without scientific measurements.

Sorry is this is redundant - but I was afraid I was confusing in my earlier posts. To be clear, the discrepancy I'm experiencing seems to be a constant distance traveled (up, then back down) - but each time it happens the travel up is a set amount less than it should be. So for example: spindle rises 100mm, comes back down and rests at 100.02. The next time it travels up and back, it rests at 100.04. Next 100.06 etc. although it's moving too fast for me to measure at the top of the axis, I'm visually seeing it after many runs of this same maneuver.

If you already had that down - please ignore it :)